Lithography and offset lithography are common methods of printing today. The terms “printing,” “marking” and “image forming” are used interchangeably in this disclosure. In a typical lithographic printing process an image transfer surface, which may be a flat plate, a surface of a cylinder or drum, a surface of a belt or the like is formed to have “image regions” formed of a hydrophobic/oleophilic material, and “non-image regions” formed of a hydrophilic/oleophobic material. The image regions correspond to the areas on the final print (i.e., the target image receiving media substrate) that are occupied by a printing or marking material such as ink to form images on the substrate. The non-image regions correspond to the areas on the final print that are not occupied by the printing or marking material. The hydrophilic regions accept and are readily wetted by surface preparation fluids, which may include water-based fluids or other compound fluids, that may be commonly referred to as dampening fluids or fountain solutions. These fluids conventionally consist of water and small amounts of alcohol and/or other additives and/or surfactants that are included to reduce surface tension of the fluids.
The hydrophobic regions repel dampening fluid and accept ink, whereas the dampening fluid formed over the hydrophilic regions forms a fluid “release layer” for rejecting ink. The hydrophilic regions of the printing plate thus correspond to unprinted areas, or “non-image areas”, of the final print.
As indicated above, in varying conventional systems, the ink may be transferred directly to a target image receiving media substrate, such as paper, or may alternatively be applied to an intermediate image transfer surface, such as an offset (or blanket) cylinder in an offset printing system. Offset cylinders are often covered with conformable coatings or sleeves with surfaces that can conform to the texture of the target image receiving media substrate, which may have, for example, a surface peak-to-valley depth somewhat greater than the surface peak-to-valley depth of the imaging plate. Also, surface roughness of the offset (or blanket) cylinder helps to deliver a more uniform layer of the printing materials, including inks, to the target image receiving media substrates free of defects such as mottle. Sufficient pressure is used to transfer the image from the offset (or blanket) cylinder to the target image receiving media substrate. This pressure transfer often occurs at a transfer nip through which the target image receiving media substrate is pinched between the offset (or blanket) cylinder and an opposing pressure member, such as an impression cylinder, that provides the pressure on the non-image side of the target image receiving media substrate.
Typical lithographic and offset printing techniques use plates that are permanently patterned, and are therefore useful only when printing a large number of copies of the same image (i.e., for long print runs), such as magazines, newspapers, and the like. These techniques are not generally considered useful in creating and printing documents in which new patterns are generated from one page to the next without removing and replacing the print cylinder and/or the imaging plate. In this regard, lithographic techniques cannot accommodate true high speed variable data printing in which the images change from impression to impression, for example, as in the case of digital printing systems. Furthermore, the cost of the permanently patterned imaging plates or cylinders is amortized over the number of copies. The cost per printed copy is therefore higher for shorter print runs of the same image than for longer print runs of the same image, as opposed to prints from digital printing systems.